I think the point was that they got greedy. They knew from the beginning of the journey on the helicopter that the load would be too heavy, there was a specific conversation about it.

I guess the bigger question is why none of them advised against this at that moment.

@Markoff said:

if they see the load it's too heavy why just not leave half just under mountain and go over twice and in the other side they can transport it again together to the boat? that was quite dumb trying to go over limit of the helicopter

Because that makes too much sense and would have defeated the whole "money is evil" theme of the movie. I think this movie must have been written by a bunch of high school kids - it just went from bad to worse.